The present invention relates generally to the operation of gas turbine engines and, more particularly, to a system and method for operation of a gas turbine engine with constant electrical power generation frequency under varying load conditions using compressor variable geometry.
Driving electric generators with gas-turbine engines is well known in the art. When using a single-spool turbine to generate alternating current (AC), the frequency of the AC output will be directly proportional to the rotation rate of the turbine because the generators are connected to the turbine's shaft either directly or through a gear box. Double spool turbines use a separate shaft with its own turbine blades to drive the generator. In either configuration, a problem arises when the load on the generators increases or decreases. A decrease in electrical load causes a drop in torque in the generator and results in the turbine rotating faster. An increase in load causes the turbine to rotate slower. These changes in rotation speed change the frequency of the AC output of the generators, which may be detrimental to the operation of the electrical load.
A number of schemes have been developed to maintain the rotational speed of the turbine with changes in load. Typically, the speed of a turbine is controlled by the amount of fuel supplied to the combustion chamber and the amount of air supplied by the air intake. Air intake is often controlled by adjustable vanes arranged in front of one or more rotating compressor blade rows. The adjustable vanes are rotated to open or closed to adjust the air flow through the various compressor blade rows. A feedback system is generally employed that measures the rotation rate of the turbine shaft, positions the adjustable guide vanes, and sets the fuel flow as needed to maintain a target rotation speed.
For example, U.S. Pat. Nos. 6,735,955 and 6,758,044 disclose a control system for positioning compressor inlet guide vanes using a normal mode schedule and an alternate schedule, where the alternate schedule is used during fast engine acceleration from low engine power. Neither schedule however, is programmed to respond to changes in load during normal operation.
In another example, U.S. Pat. No. 6,164,057 discloses a reserve capacity controller that operates a gas turbine such that a desired reserve power capacity is maintained by using the inlet guide vane angle as an indicator of the reserve levels. The actual inlet guide vane angle is continuously compared to an intended inlet guide vane angle that corresponds to a desired reserve capacity. When the actual inlet guide vane angle differs from the intended inlet guide vane angle, the controller adjusts the fuel flow to the gas turbine to reset the actual inlet guide vane angle.
Such feedback systems suffer from a drawback, however. They generally are unable to respond quickly to sudden large increases in load on the order of hundreds of kilowatts, though they have historically done well with sudden decreases in load. It may take anywhere from 3 to 15 seconds for a typical feedback system to restore a sudden drop in frequency. For some AC applications, this will be an acceptable delay, but for frequency-sensitive applications, this presents a substantial problem.
One such frequency-sensitive application is aircraft electrical power generation. Aircraft electrical power generation requirements become even more demanding with the integration of high power multi-mode phased array radar systems into new or existing aircraft designs. While the radar operation may not be specifically dependent on the frequency of generator output, the power management system and other aircraft components are certainly affected by the wide range of loading levels required by the radar. The multi-mode phased array radar aggravates the frequency problem by randomly switching its operating modes. Each radar mode demands a different power level and thus creates a dynamic loading profile with a wide range of load levels. Operation outside of the required frequency range leads to excessive current levels in generator windings and inaccurate operation of other aircraft electrical components. Therefore, operation below or above target frequency limits demands that the electrical generators be shutdown to prevent electrical damage. However, shutdown of electrical power generation prevents acceptable use of the aircraft. Therefore, an effective aircraft power generation system must be designed to allow rapid, random variation of electrical loads from either the radar or supporting aircraft electrical systems.
What is needed is a system that minimizes frequency variation when exposed to rapid and random load changes which also recovers a desired turbogenerator rotational speed following sudden and substantial changes in electrical load.